The goal here is to get your horse to respond to your rein cue, which is obviously not doing! Lol
What you shoudl try and work on is to lighten him up at a stand still, then a walk. At a stand still, apply pressure to the reins to get him to back. The moment he takes a step, release the pressure. Do the same thing from a walk to a halt. The moment he stops release pressure.
You want him to understand that the moment he responds to you, he gets rewarded by removing the pressure that you are applying.
Yes, you do want to sit in your saddle, but not back. If you are sitting forward, you are applying forward "pressure" encouraging him to walk/trot faster. If you sit deeper in your saddle, you are encouraging him to slow down a bit.
Try being as light as you can on him, and increase your pressure when you get 0 results. Some horses have hard mouths and its harder for them to respond as quickly because they are so used to harsh pressure.
Hope this helps! P.s. Circles and patterns are a great way to get his attention/make him think and also to cut down some of the speed.